U.S. Pat. No. 4,608,349 (Kerko et al.) describes glasses that have a photochromic potential, and that are characterized by a Li.sub.2 O:Na.sub.2 O molar ratio of at least 9:1. These glasses are designed to produce photochromic lenses that have refractive indices corrected for ophthalmic use.
Such lenses darken to a value less than 35% when exposed to solar simulated radiation, and rapidly fade when the exposure is removed. Composition-wise, the glasses consist essentially of, as calculated in weight percent on the oxide basis:
______________________________________ SiO.sub.2 54-58% Ag 0.20-0.33% B.sub.2 O.sub.3 18-22% Cl 0.30-0.50% Al.sub.2 O.sub.3 7-8% Br 0.40-0.14% Li.sub.2 O 3.75-4.5% CuO 0.007-0.012% Na.sub.2 O 0-1% PbO 0-0.08% TiO.sub.2 0-2% Sb.sub.2 O.sub.3 0-0.20% ZrO.sub.2 12 2-4.5% K.sub.2 O 5.5-7.5% ______________________________________
wherein the mole ratio of Li.sub.2 O:Na.sub.2 O is =&gt;9:1.
Glasses that are potentially photochromic require a heat treatment to develop photochromic behavior. The -349 patent teaches heat treatments at temperatures in the range of 550-660.degree. C. for times ranging from 10-30 minutes. However, the exemplary heat treatments, set forth in the patent, range from 625-660.degree. C. for 30 minutes.
A heat treating cycle in the range of 625-630.degree. C. for thirty minutes is employed commercially to produce minus power lenses from these glasses. Such lenses have thicknesses in the range of 1.5-1.8 mm., and are used to treat patients having myopia.
Plus power lenses are needed for patients afflicted with hyperopia (far sightedness). These lenses require a center thickness greater than 2 mm. The same glass and heat treatment can be used to produce the thicker, plus power lens. However, the thicker lens may become too dark for some applications in cold weather. For example, at 0.degree. C., the glass darkens to 2-3% transmittance.
It is, of course, possible to produce plus power lenses in other types of photochromic glass with a standard heat treatment. However, such glass lenses would not have the desirable properties obtained with the glasses of the Kerko et al. -349 patent. Further, such practice would mean frequent composition changes if one melting unit were employed. The alternative would be to use separate melting units, an alternative that is equally unattractive.
Ophthalmic suppliers are reluctant to carry a line of lenses unless they can carry a full line of both plus and minus power lenses. Thus, there is a reluctance to carry the line of minus power, glass lenses of the Kerko et al. -349 patent in the absence of a corresponding line of plus power lenses. This has led to the need for a convenient means of producing a full line of lenses in one of these glasses.
It is a basic purpose of the present invention to meet this need.
A further purpose is to provide a full line of ophthalmic glass lenses employing a single glass having the several characteristics set forth in the Kerko et al. -349 patent.
Another purpose is to provide a fast fading, plus power, ophthalmic lens.